The present invention generally relates to a programmable logic controller, and more particularly to a programmable logic controller that uses micro-electromechanical system based switching devices to provide a digital input and output interface.
Programmable logic controllers (“PLCs”) are widely used for automation and control. These include industrial automation, factory automation, building automation and the like. PLCs are typically programmed by users with a control program to implement their desired functionality. The control program will be written in a programming language, such as ladder logic for example, to execute the desired control. For example, a PLC may monitor input conditions such as motor speed, temperature, pressure, volumetric flow and the like. The control program is stored in memory within the PLC to instruct the PLC on what actions to take upon encountering certain input signals or conditions.
PLCs do not usually connect directly with the devices that they control. An interface module, or input/out module (“I/O module”) is used to provide the necessary connections and adapt the signals into a usable form for both the device and the PLC. The I/O module further protects the PLC from abnormal electrical faults such as short circuit conditions. I/O modules are typically mounted into respective slots located on a backplane board in a PLC. The slots are coupled together by a main bus that couples any I/O modules to a central processing unit (“CPU”).
Two types of devices that are typically used to interface the PLC with higher electrical power applications are electromechanical relays and solid-state devices. These devices allow the PLC that is a digital logic device that operates using a 5 VDC circuit, to control devices that operate at higher voltages and currents, 24 Vdc or 120 Vac for example.
A relay is an electromechanical switch that uses an electromagnet to operate. To switch on/off current in electrical systems, a set of contacts may be used. The contacts may be either in an open position, resulting in the stopping of current flow, or in a closed position that allows current flow. A coil within the relay is energized creating a magnetic field that acts on an armature. The armature is mechanically connected to the contact that causes the contact to move under the magnetic field created by the coil. The movement either makes or breaks the electrical connection created by the contact. Relays provide advantages of being highly reliable and wide varieties are readily available. Thus the correct relay may be easily matched with the application. However, these electromechanical switches tend to be slow to operate and also need to be designed or selected for a particular power level. Since the relay operates at only one power level, the scalability of the device is limited. Further, relays do not provide protection for themselves or the circuits that they drive.
As an alternative to slow electromechanical switches, fast solid-state switches, such as a bipolar junction transistor or a metal-oxide-semiconductor field-effect transistor (“MOSFET”) for example, have been employed in direct-current applications at low power levels. These solid-state switches switch between a conducting state and a non-conducting state through controlled application of a voltage or bias. For example, by reverse biasing a solid-state switch, the switch may be transitioned into a non-conducting state. However, because solid-state switches do not create a physical gap between contacts as they are switched into a non-conducing state, they experience leakage current. Since there is no physical gap, the electrical protection provided by the solid-state switch is limited. Further, similar to the relay, the solid-state switches are sized for a particular electrical power level and therefore have limited scalability.
While existing I/O module technology is adequate for its intended purposes, there exists a need in the art for a PLC having an I/O module that is provides scalability and electrical fault protection that overcomes these drawbacks.